Pressure sensitive adhesives are a class of materials that adhere to a variety of solid surfaces upon light application of pressure. Pressure sensitive adhesives are used in many applications such as paper labels, graphics, tapes, medical bandages and, increasingly, for plasticized vinyl. For plasticized vinyl, the pressure sensitive adhesive must be able to retain its adhesive and cohesive properties over an extended period of time both at room and elevated temperatures. However, in practice, the migration of monomeric plasticizer causes deterioration of the pressure sensitive adhesive properties by making it sticky and gummy, i.e., loss of cohesive strength, or by creating a slick plasticizer film that causes interfacial adhesion failure.
Several approaches have been reported in the literatures to improve the plasticizer resistance of pressure sensitive adhesives. The majority of the literature concentrates on solution and water borne adhesives for vinyl applications. One method to improve plasticizer resistance of pressure sensitive adhesives is by partial crosslinking. UV cured solution acrylics have been shown to improve plasticizer resistance, e.g., Bogaert et al., U.S. Pat. No. 5,079,047. Water borne adhesives made by emulsion polymerization of ethylene vinyl acetate and acrylics have also been shown to improve plasticizer resistance, e.g. , Mudge, U.S. Pat. No. 4,961,933; Iovine, U.S. Pat. No. 4,822,676; and Graziano et al., J. Plastic Film and Sheeting, 2, 95 (1986) . Heat cured natural rubber solution adhesive used for high temperature masking tape application also show good plasticizer resistance.
These prior art methods are generally not practical for use with conventional hot melt thermoplastic adhesives. Hot melt adhesives are 100% solid adhesives that usually coated onto a substrate in a low viscosity molten form (&lt;100,000 cps and at 350.degree. F.) using hot melt coating equipments such as roller coater, slot die, gravure, etc. They are different from a calendared coated product or skimmed or frictional product commonly practiced in rubber and plastics industry. The plasticizer resistance of hot melt adhesives, has been improved by radiation curing of styrenic block copolymers as disclosed in Hansen et al., U.S. Pat. No. 4,133,731, or moisture curing of urethane reactive hot melts as taught by Davis, in "Urethane Reactive Hot Melts", a paper presented at the Adhesives and Sealants Council Meeting, Philadelphia, Pa., Apr. 1, 1987. These methods, however, require special equipment and are very expensive to carry out. Because of their high cost and environmental/health effects, these prior art methods have generally been practiced in house by the captive manufacturers. Moreover, prior art methods for in situ curing of the adhesive during mixing/preparation lead to poor heat stability or gelling, e.g., Korpman, U.S. Pat. No. 4,028,292.
It is the object of this invention to provide a 100% solid thermoplastic hot melt adhesive with improved plasticizer resistance without the necessity of post curing step.